The leading cause of lower back pain arises from rupture or degeneration of lumbar intervertebral discs. Pain in the lower extremities is caused by the compression of spinal nerve roots by a bulging disc, while lower back pain is caused by collapse of the disc and by the adverse effects of articulation weight through a damaged, unstable vertebral joint. One proposed method of managing these problems is to remove the problematic disc and replace it with a prosthetic disc that allows for the natural motion between the adjacent vertebrae (“a motion disc”).
U.S. Pat. No. 6,368,350 (“Erickson”) discloses a three-piece motion disc providing two articulation surfaces. The disc comprises a first piece having a curved surface, a second piece having a flat surface, and an intermediate piece having a corresponding curved articulation surface and a corresponding flat articulation surface. In many embodiments, the translation freedom of the intermediate piece is limited by a raised lip integrally formed around the edge of a flat surface upon the lower piece. Erickson teaches that the overall height of the device is varied by increasing or decreasing the thickness of one or more of the first, second or intermediate pieces. Erickson teaches that known methods for insertion of intervertebral prosthetic devices can be used for insertion of its device. Lastly, Erickson teaches that a variety of materials can be selected as materials of construction for the components of its device, including metals, polymers, and ceramics, and specifically teaches preferred combinations including metal-metal or metal-plastic combinations.
In each of Erickson's embodiments having a peripheral raised lip, the height of the core member appears to exceed the distance between the peripheral raised lips of the opposing endplates. Accordingly, the core member can not be inserted between the prosthetic endplates without overdistracting the disc space.
Erickson does not teach an open ended channel for inserting the intermediate piece between the prosthetic endplates, nor an additional component for retaining the intermediate piece upon the flat surface. Erickson does not teach piecemeal insertion of the device into the disc space. Erickson does not teach a metal-ceramic articulation interface.
U.S. Pat. No. 5,676,701 (“Yuan”) discloses, in one embodiment, a motion disc having a single articulation surface. This device includes a first component whose inner surface comprises a concave inner portion having a 3600 circumference and a convex peripheral portion, and an opposing second component whose inner surface comprises a conforming convex inner portion and a convex peripheral portion. The convex/concave contours of the opposing inner portions forms a ball-and-socket design that allows unrestricted pivotal motion of the device, while the opposing convex peripheral contours allow flexion/extension bending motion in the range of about 20-30°.
In another embodiment, Yuan discloses a device having two articulation interfaces, wherein one of the above-mentioned components is made in two pieces having opposing flat surfaces that form a translation interface to further provide the prosthetic with a certain amount of translation. See FIG. 9 of Yuan. Yuan discloses that the translation-producing pieces can be fitted together mechanically, via shrink-fit, or by welding methods.
However, Yuan does not disclose an open-ended channel for fitting the translation producing pieces.
U.S. Pat. No. 5,507,816 (“Bullivant”) discloses a three-piece motion disc providing two articulation interfaces and comprises an upper piece having a flat lower surface, a middle spacer having a flat upper surface and a convex lower surface, and a lower piece having a concave upper surface. The articulating convex and concave surfaces form an articulating interface that allows pivotal motion, while the flat surfaces form a translation interface that allows translational motion. Bullivant further teaches that the natural tension of the vertebrae ensures that the vertebrae are biased together to trap the spacer in place, and that the 90° extension of the convex and concave surfaces virtually eliminates any chance of the spacer escaping from between the plates under normal pivotal movement of the vertebrae.
The Bullivant device does not possess any channel for retaining the middle spacer within the device. Accordingly, it is prone to disengagement.
In each of the Erickson, Yuan, and Bullivant designs, the core member has a flat translation surface and a curved articulation surface.
There are currently two primary competitive artificial disc replacement devices on the market that are designed for the lumbar spine.
The first device has two articulation interfaces and comprises three components: an inferior endplate, a superior endplate, and a core. Both the inferior and superior endplates are metal and have raised bosses with concave spherical surfaces in the center. The core is plastic and has convex surfaces on both the top and bottom which are surrounded by raised rims.
However, this device does not have an open ended channel for inserting the core between the endplates. Related devices are disclosed in U.S. Pat. Nos. 4,759,766; 5,401,269; and 5,556,431.
In each of the devices disclosed in these three patents, the core member has either two concave surfaces or two convex surfaces.
The second device has a single articulation interface and comprises three components: an inferior endplate, a superior endplate, and a plastic insert. The inferior endplate functions as a baseplate and has a sidewall forming an open ended channel for reception of the insert. The inner surface of the inferior endplate provides only stationary support for the insert and does not have a motion surface. Since the plastic insert is designed to be locked securely into place within the inferior endplate, the inferior surface of the insert is not a motion surface. The superior surface of the insert includes articulation surface for articulation with the superior endplate. The superior endplate has an inferior articulation surface that articulates with the superior motion surface of the plastic insert, and a superior surface designed for attachment to a vertebral endplate. A related device is disclosed in U.S. Pat. No. 5,314,477.
The second device does not have two articulation surfaces. The second device relies upon downward-extending flexible tabs disposed upon the insert to keep the insert within the open-ended channel. These tabs eliminate any ability for the insert to translate with the adjacent endplate surfaces.
French Published Patent Application No. 2,730,159 (“Germain”) discloses a motion disc in which the core member has one convex and concave surface. Germain further teaches that the radius of the upper curved surface (3a) of the core member is less than the radius of the lower curved surface (3b) of the core member.
Therefore, there is a need for a motion device having two articulation interfaces that allows for initial insertion of the prosthetic endplates into the disc space and then insertion therebetween of a core member having two articulation surfaces.